How Dark Matter Messes with Our Galaxy [Video]

This video shows the spiral shape of our galaxy and two of its small satellite galaxies, the Large and Small Magellanic Clouds (to the left). The satellites orbit the main galaxy and, in the process, trigger slow-motion waves in it. On a human time scale, those waves look like a static warp in the galactic outskirts. The strange thing is that the satellites are too lightweight to have such a dramatic effect. Astronomers recently demonstrated that their gravity is greatly amplified by dark matter (which this video does not show -- it is, after all, dark).

The Magellanic Clouds are not the only satellites of the Milky Way. Astronomers have counted some two dozen. This video shows their three- dimensional position relative to the plane of the galaxy, where the sun and most other stars lie. Here, the mystery is why there are not more satellites: by rights, our galaxy should have hundreds. The Sloan Digital Sky Survey, which scanned the region shown by the pinkish cone, partially filled in the gap by finding another dozen satellites. They are extremely dim and composed mostly of dark matter. The rest of the satellites out there may be completely invisible.